Method of producing cold rolled steel structural members



R. FRANKS Sept. 26, 1944.

METHOD OF PRODUCING com) ROLLED STEEL STRUCTURAL MEMBERS Filed June 17,1941 2 Sheets-Sheet 1 'HME lN HOURS INVENTOR RUSSELL FRANKS ATORHEYSept. 26, 1944. FRANKS 2,358,799

METHOD OF PRODUCING COLD ROLLED STEEL STRUCTURAL MEMBERS Filed June 17,1941 2 Sheets-Sheet 2 Y s e 7 X 8 o YIELD STRESS g 130,000 PSI. o o In man J o 8 115 .J o o o N 1 iE- 2 m- .J O 8 o 7/ 2 PROPORTIONAL LIMIT50,000 LB. PER SQUARE INCH PROPORTION/XL LIMIT 23,500 LB.

PER SQUARE INCH SECTION OF SAMPLE I=0.0I47 SQJN. SECTION OF SAMPLE2=0.0I5I SQJN.

O l 2 3 4 5 6 7 STRAIN THOUSANDTHS OF AN INCH ELONGATION PER INCHINVENTOR (ZINCH GAGE LENGTH) BY Patented Sept. 26, 1944 METHOD OFPRODUCING COLD ROLLED STEEL STRUCTURAL MEMBERS;

Russell Franks, Niagara Falls, N. Y., assignor to Electro MetallurgicalCompany, a corporation of West Virginia Application June 17, 1941,Serial No. 398,400

6 Claims. ((3. 148-12) This application is in part a continuation of myapplication Serial Number 297,229, flied September 30, 1939, which inturn is in part a continuation of my application Serial Number 247,210,filed December 22, 1938.

The present invention relates to cold rolled and otherwise cold workedaustenitic type chromium steels. The large demand for improved steelsfor building strong, light structures, notably in the field oftransportation, has been met in part by cold rolled corrosion-resistantsteels containing chromium within the limits of 12% and 30% togetherwith enough austenite-promoting elements to make the steels at leastpredominantly austenitic. Steels of this type may be cold rolled to atensile strength of more than 160,000 pounds per square inch by areduction in thickness of, say, 10% to 6 of the original thickness. Thecold rolling does not seriously afiect the resistance of the material toatmospheric corrosion and weathering, and it is therefore safe to usethin, light-weight sections of the strengthened material in structuresexposed to the weather.

A shortcoming of these steels, which has hindered the exploitation oftheir exceptionally high tensile strength, lies in their relatively poorelastic properties revealed, for instance, by the low value ofproportional limit obtained by conventional testing methods. Indeed, ithas been suggested that the cold-rolled material does not have a trueproportional limit and that it begins to elongate permanently as soon asa small load is applied. It is likewise difilcult to determine themodulus of elasticity of the material from experimentally determinedstress-strain values. Designers customarily assume a modulus ofelasticity of about 25 million pounds per square inch construct a linerepresenting this modulus on an experimentally determined stress-straindiagram, construct a second line parallel to this assumed modulus lineand spaced therefrom by a distance equivalent to a strain of 0.2% in twoinches or 0.002 inch per inch of gage length, and take as the yieldstress the point of intersection of the second line with thestress-strain curve. The "yield strength? calculated from the yieldstress obtained in this manner is considerably below the measuredstrength at the stress that will cause the steel to rupture, and may be,for instance, as low as 100,000 pounds per square inch. The yield stressvalue is of course outside the elastic limit, but is high enough topermit of fairly accurate determination.

Another shortcoming of the cold rolled austenitic type steels is thattheir strengths, meas- 'ured by conventional methods, are diflerent intension than in compression, and diflerent when measured along thedirection of rolling than when measured transverse thereto. In a typicalinstance, the following yield strengths (0.2% set) maximum tensilestrengths, and buckling strengths in compression, in pounds per squareinch, were measured on samples oi the same cold rolled austenitic typechromium nickel steel, respectively in the direction 0! rolling (L) andtransverse thereto (T):

Thus, although the tensile strength as ordinarily measured was 178,000pounds per square inch, designers could take but little advantage oi.this property because lower stresses caused permanent large deformationin tension and compression.

Although the superior resistance 01' the austenitic type chromium steelsto corrosion gives them an advantage over the strong light metal alloys,for instance the Duralumin." "Hiduminium, and "Birmabright groups ofaluminum alloys, their greater specific gravityis a handicap which canbe overcome only by the development of relatively very great strengthwhile maintaining satisfactory toughness, ductility, and fatigueresistance. Heretofore, it has not been Possible to produce steels whichnotably surpassed the light metal alloys on a strength to weight ratiobasis and were otherwise wholly satisfactory. For several years therehas been a large'demand for improved high strength corrosion resistantsteels for aircraft and other structures, for instance for aircraft usedin over-water service and therefore exposed to corrosive waters.

It is an object of this invention to improve the elastic properties (asevidenced for instance by such properties as the yield point, yieldstrength, and modulus of elasticity) of cold rolled and otherwise coldworked (e. g., cold drawn. cold twisted or bent, cold pressed)austenitic type corrosion resistant chromium steel without verymaterialiy altering its toughness, ductility, ultimate tensile strength,ultimate compressive strength, hardness, or resistance to corrosion. v

Further objects of the invention are to impart to cold worked steel ofthe type described a definitcly determinable and high proportional limitand to raise its yield strength as determined in the manner describedabove; to provide such a steel having a definitely determinable highmodulus of elasticity substantially the same as that of a fully annealedsteel of the same composition; and to increase the fatigue endurance ofsuch a steel. Other objects of this invention are to provide a methodfor raising the proportional limits to above 30,000 pounds per squareinch and the yield strengths to above 120,000 pounds per squar inch, andpreferably above 150,000 pounds per square inch, both in the directionof rolling and in a direction transverse thereto, and both incompression and in tension; to provide a method for making more nearlysimilar the tensile and compressive elastic properties; and to provide amethod for making the elastic properties more nearly uniform in alldirections throughout the steel section.

Further objects of the invention are a method of producing a cold rolledaustenitic type chromium steel structural member, characterized by aproportional limit at least 45,000 pounds per square inch and a yieldstrength (to 0.2% set) upward of 160,000 pounds per square inch, intension and in compression, in the direction of rolling and transversethereto; by a calculated modulus of elasticity substanti' lly the sameas that of fully annealed steel of the same composition; by an improvedresistance to fatigue; by high toughness, ductility and resistance tocorrosion; by a metallographic structure substantially free frommicroscopically visible precipitated carbides or similar compounds; andby a bright cold-rolled surface.

The objects of the invention are attained by a method which comprisessubjecting cold rolled (or otherwise cold worked) austenitic typecorrosion resistant chromium steels to a heat treatment for controlledperiods of time at temperatures within the range of 200 F. to 625 F.,preferably within the range of 250 F. to 550 F.

The invention will be more particularly. described with reference to theaccompanying drawings, in which:

Fig. 1 is a diagram on semilogarithmic coordinates, indicating theranges of time and temperature to be used in the practice of theinvention;

Fig. 2 is a diagram on rectangular cartesian coordinates, of theobserved stress-strain curves (S1 and S2) of an austenitic type ofsteel, respectively before and after subjection to the treatment of theinvention. Also shown in this figures are the lines (M1 and M2) fromwhich moduli of elasticity may be determined and the line (D1 and D2)from which yield stresses may be determined, and

Fig. 2a is a continuation of the upper portion of Fig. 2. The line X-Yin Fig. 2a corresponds with the line X-Y in Fig. 2.

It is essential to the successful application of the method of theinvention that the time of treatment be properly related to thetemperature of treatment. For any given temperature within the range of200 to 625 F. there is a minimum time of treatment and a maximum time oftreatment, the optimum time being between the minimum and maximum anddepending on such factors as the composition of the steel, the amount ofcold work that has been done on the steel, the tensile and elasticproperties desired, and

economic factors. A treatment for a time shorter than th minimum isineffective to produce a useasssrzeo iul improvement in elasticproperties. A treatment for a time longer, or at a temperature higher,than the maximum often produces undesirable effects. including one ormore of such eilects as a decrease in strength, for instance by anannealing (softening) action; the formation of a surface scale or a darksurface discoloration; a decrease in resistance to surface corrosion; asusceptibility to intergranular corrosion; a loss of toughness; anincreas in hardness, as by precipitation of carbides or other compounds;or a coarsening of the grain structure. When the time of treatment isproperly related to the temperature, none of these undesirable effectsoccurs and, indeed, there is not even a metallographic change observableunder the microscope at magnifications'of 2000 diameters.

The correct relationship of time and temperature is indicatedgraphically in Fig. 1. Suitable times and temperatures are thoseincluded between lines A, B, C, and D, and preferred times andtemperatures are included between lines E, F, G, and D. Time temperaturecombinations above the line A are such as will produce undesirableeffects while those below the lines B and C will not bring about thedesired improvements.

The lines C and G of Fig. 1 may be represented by the formula: (H) 10=K, wherein H is the time in hours, T is the temperature in Fahrenheitdegrees, and a. is 0.002588. The value of K is 83 in the case of line C,and 150 in the case of line G.

Although periods of time somewhat longer than hours, say up to 200hours, may be used at the lower temperatures without serious effects,such long heating times are not commercially attractive.

The use of temperatures between 475 and 625 F. may in some instancesproduce a slight surface discoloration, not ordinarily displeasing ingeneral appearance. Any discoloration may be removed very quickly withacid or an acid pickling solution without damaging the cold rolledsurface finish. If desired, an inert or bright annealing atmosphere maybe used to avoid or minimize discoloration of the steel, therebyaffording somewhat greater latitude to the times that may be used atthese higher temperatures.

The method of the invention may be applied to advantage to a widevariety of cold worked austenitic type corrosion resistant steelscontaining about 12% to 25% chromium. Such steels may contain 4% to 16%of nickel or manganese or mixtures of nickel and manganese; up to 0.3%,or somewhat more, carbon; with other elements, such as silicon,nitrogen, copper, columbium, tungsten, and molybdenum, customarily oroccasionally added to austenitic type chromium steels. Preferably, thechromium content is between 15% and 19%, and the total amount of nickeland manganese does not exceed 10% and is at least 5%. If the steelcontains a small amount of molybdenum or tungsten, the total nickel andmanganese percentage is preferably at least 8% to improve the hotworking properties; if the amount of such elements is large, the totalnickel and manganese percentage is preferably at least 10%. Columbiummay be present, provided that it is no more than 8 times the carboncontent, 1. e., provided substantially all of it is combined with thecarbon to form a compound insoluble in the steel. It is preferred thatthe amount of silicon be less than 3%, and in most cases less than 1%. Asuitable amount of nitrogen is 0.01%

to 0.3%, although the amount of this element may on occasion be as muchas 0.4%. A suitable amount of copper is 0.25% to 2.5 and of molybdenumor tungsten or both, 1% to 5%.

Experimentally determined effects of the ap- TABLE TYPICAL Erncrs orHEATING 24 HOURS AT 200 C.

(392 F.) AND AIR CooLINc 0N SraaL CoLnplication of the method of theinvention to, sev- ROLLED sY VARIOUS PERCENTAGES or REDUCTION eraltypical steels oi the kind described appear in Tables A to D. In thesetables are given the STEEL AMPLE 00% INCH THICK chemical analyses of therespective steels, the remainder in each case being iron and the usualPercentage reducimpurities in insignificant amounts; the obggggg servedproportional limit in tension, in thousands of pounds per square inch;yield stress in 20 30 4o tension, in thousands of pounds per squareinch, calculated as indicated above; the observed maxipmmmommmmsmued m31 22 mum stress in tension, in thousands of pounds 52 g g ggg gfifiheating. 42 40 50 per square inch; .and the observed percentage w m n-}ifi::" 1 3% 13 elongation in a two inch gage length. These gg%figgggggfig l i- 1% 21 13 tensile tests were made on a Standard tensilePercentelongation in2inohes 1351611551. 30 18.5 13 testing machine usinga Berry gage and strip QZg fl E F e :g e i 30 18 8 samples, of therespective thicknesses indicated Rockwell 0" 11311011652810; him ting:-i in the tables, tested in the direction of rolling. The samples wereprepared from hot wrought STEEL N0. 1 SAMPLES 0.00 INCH THICK blanks bycold rolling to final thicknesses. The reduction in thickness by coldrolling is expressed P ti 1r it n d as a percentage of the thickness ofthe blank gg gg ggg gg gi ggg 3g g :3 before cold rolling.Yiedstressasrolled 10 12g 1 0 fieldlgtress alter heatillll% 118 159 197ax umsressasro e 169 190 203 TABLE A :1E\;IaXilIl1lI;StfeStS 8ft 8l'lzli iiltifiig l l 1 162 183 213 ercen eonga l0l1111 nc csasro e 31 2512 ANALYSES OF STEELS IN TABLES B To D Per cent elongation in 2inchesafter heating... 25 8 3o Rockwell C hardnessasroiled 36 41 Rockwell 0"hardness after heating 38 43 47 Analysis (remainder Fe and incidentalimpurities) swelNo' For For Pet Per Per Per Per TABLE D a a a a a a as rn 5 TYPICAL EFFECTS or VARlOUS TIMES OF HEATING AT 0 O 7'75 M4 M5 0.11M6 None 200 C. (392 F.) AND AIR COOLING 2 3'2? 8 1 190 X1 3 t 11v 1 1003 k W 7.62 0.42 M2 Q 12 0.12 None e o. samp e in. t 1c 30% reduction1.54 0.50 0. 25 0.07 0.04 None 1! l in 2.20 9.09 0.46 0.11 0.12 None 0it: it 8'21 858 8'62 0116 7.69 Low Low 0.07 Low None Tune of heatmghrs'0 s 16 a4 04 72 TABLE B 45 Pro rtional limit 14.4 56 62 49 53 56 TYPICALErrncrs or HEATING AT 200 C. (392 F.) giei ziilstrgssnufin- 106 152 106150 163 AND Am COOLING. ALL SAMPLES 0.03 INCH ens engt 133 190 181Percentelongationin2inches 18.5 16 20 18 20 18.5 THICK, 30% REDUCTION BYCOLD-ROLLING I. teen our eatin at 200 C. 392 F.)

h s h g steel No. 8, sample 0.02 in. thzck, 30%-reduction ld-r lliStcelnumber by co 0 M 1 2 3 4 Time of heating, hrs.Proportionallimitasrolled 14.4 22 23 30 Pro ortionallimit alter heating.62 47 59 0 1 168 Yiedstressasrolled 123 126 132 130 Yield stress afterheating... 152 153 166 148 Maximum stressasro11ed 199 185 176 164 Proortional limit 20 25 30 Maximum stress after heating 183 182 188 169 Yidstress 128 143 Per cent elongationin2inch.asro11ed 18.5 17 8 20.5Tensile strength 162 157 156 Per cent elongation in 2 inch. aiter 60Percent elongationin2inches 23 25 25 heating 20 18.5 5 18 II. Sixty-fourhours heating at 200 0, (392 F.) Tables E and F comprise data obtainedon two representative. steels, before and after applying Steel number 05the heat treatment of the invention, in not only the tensile testsreferred to in connection with 5 6 7 Tables B to D but also compressiontests on cold iormed cylindrical tube samples having an insidegroportional 11ml: alsrolgsdv 24 g; 2; di meter of 1.5 inch; a wallthickness of 0.035

to ort one imi ater ea 67 5 yiallfismss toned 119 m 70 inch, and alength of 2 inches, the ends be ng 1 910 stress 11m heatillllgd 188 13%supported in Woods metal to avoid bending P 8 stresses. Samples weretested respectively in the Maximum stress after heatin 180 183 Per centelongationin2ineh as rolled.-. 11 11.5 18.5 direction that the steel wasrolled on and alternating 8 18 in the direction transverse thereto(Trans). The

75 tensile yield strengths were determined as descrlbed herein above.Yields in compression were measured with Huggenberger gages, those intension with Berry gages.

TABLE E Tests of steel containing 18.45% or; 8.79% Ni, 0.5% Mn, 0.55%Si, 0.10% C, 0.04% N,

remainder Fe; strip section reduced 35% by cold rolling. Tensile samples0.03 znch thick As cold After 72 hr. rolled at 392 F.

Proportional limit in tension, Long 45, 000 55, 000 Proportional limitin tension, Trans 48,000 000 Proportional limit in compression, Long 17,100 47, 600 Pro ortional limit in compression, Trans" 42, 700 72, 000Yie 11 strength in tension, Long 132, 000 154, 000 Yield strength intension, Trans. 130, 000 148, 000 Yield strength in compression, Long-94,100 120, 000 Yield strength in compression, Trans. 150,200 171 000Maximum stress in tension, Long 155,300 173, 400 Maximum stress intension, Trans 166, 200 178, 700 Buckling stress in compressgon, Long151, 800 158, 000 Buckling stress in compression, Trans.. 83, 11 193.000

TABLE F Tests of steel containing 17.15% Cr, 7.17% Ni, 1.32% Mn, 0.34%Si, 0.11% C, 0.05% N, remainder Fe; strip section reduced 35% by coldrolling. Tensile samples 0.035 inch thick As cold Alter 72 hr. rolled at392 F.

Proportional limit in tension, Long 45, 000 58, 000 Proportional limitin tension, Trans 46, 000 61, 000 Proportional limit in compression,Long 39, 700 62, 000 Proportional limit in compression, Trans. 47, 00065,000 Yield strength in tension, Long 162, 000 181, 000 Yield strengthin tension, Trans 140, 000 172, 000 Yield strength in compression, Long-146, 000 163, 000 Yield strength in compression, Trans 185,000 201, 000Maximum stress in tension, Long 196, 000 198, 000 Maximum stress intension, Trans. 201, 000 202, 000 Buckling stress in compression, Long-184, 500 201, 200 Buckling stress in compression, Trans... 214, 300 218,500

Inspection of Tables A to F reveals that the improvement in propertieswrought by the heat treatment of this invention is greater, the higherthe percentage reduction of section; that the improvement is greater inthose steels which are less stably austenitic than an 18% chromium, 8%nickel steel; and that the highest tensile and compressive strengths areto be obtained by a cold reduction of more than 30% of a steel lessstably austenitic than 18-8, followed by the heat treatment of theinvention.

Further research has shown that to achieve, with satisfactory ductility,in tension and compression, both in the direction of rolling andtransverse thereto, a proportional limit at least 45,000 and up to75,000 pounds per square inch, and a yield strength (0.2% set) upwardsof 160,000 and up to 225,000 pounds per square inch, in a.chromium-nickel steel containing between and 19% chromium, it isnecessary to keep the sum of the chromium and nickel percentages between21% and 26%, the manganese content below 3%, the silicon content below0.75%, and the carbon content between 0.01% and 0.2%, the nitrogencontent between 0.01% and 0.2 and to cold roll or otherwise cold workthe steel to an extent corresponding to a reduction of section between30% and 60%.

Although this invention does not depend on any theory, it seems probablethat the high strength imparted to autenitic type chromium aasaveosteels is a result chiefly of three phenomena: cold working ofaustenite, cold working of ferrite if any is present, and the alteration'of some austenite to a hard pseudo-martensite. If the steel compositionis such that the austenite is relatively stable, most of the increasedstrength imparted by cold work is due to internal straining of austenite(and ferrite if such is present), but if the austenite is unstable ahard pseudo-martensite is produced by cold work, and the increase instrength imparted by the martensitic constituent is greater than isattainable by the moderately great cold straining-of austenite orferrite imparted by rolling mills of conventional capacities.

Figs. 2 and 2a. illustrate graphically some of the eflects produced bythe method of the invention. In preparing these figures, steel number 5was cold-rolled to strip 0.03 inch thick by a 30% reduction inthickness. One sample of this strip, 0.0147 square inch incross-sectional area, was then pulled, without heat treatment, in astandard hydraulic tensile testing machine, using a Berry gage and a 2inch gage length. The unit elongations at observed increments of stresswere then plotted, giving the dots appearing on stressstrain line S1. Astraight line (M1) was then drawn through as many dots on S1 aspossible. The slope of the modulus line M1 indicates the "calculatedmodulus of elasticity of the sample, in this case 27.2 million poundsper square inch. The point at which the stress-strain curve S1 deviatesfrom the modulus line M1 indicates the proportional limit, 23,800 poundsper square inch. A deviation line D1 is drawn parallel to the modulusline M1, spaced therefrom by a distance representing a strain of twothousandths of one inch per inch; theintersection of this line D1 withthe stress-strain line S1 indicates the yield stress, 130,000 pounds persquare inch.

Still referring to Figs. 2 and 2a, a second sample of the same strip washeated at 392 F. for 72 hours and air-cooled. The treated sample, of

the preceding paragraph. The stress-strain line- S2 was therebyobtained, and from this the modulus line M2 and the deviation line D2.The modulus line M2 indicates a. proportional limit of 50,000 pounds persquare inch and a modulus of elasticity of 28 million pounds per squareinch, substantially the modulus of fully annealed material of the samechemical composition. The intersection of the deviation line D2 with thestress-strain line S2 indicates a yield stress of 176,000 pounds persquare inch.

There is some evidence, obtained by X-ray diffraction methods, .toindicate that the heat treating method of this invention acts to releaseinternal stresses within the crystalline structure of the steel without,apparently, producing the results of the known stress-relieving anneal.The absence of a. Visible precipitate, and the enhanced uniformity ofproperties throughout a sample of steel, would also indicate that themechanism of the heat treating method of the invention is through arelief or redistribution of internal stress rather than through theformation of a precipitate. But the invention is not limited to anytheory.

The method of the invention also improves the fatigue endurance of thesteels of the class described. The data set forth in Table G areselfexplanatory and indicate such improvement.

TABLE G4 Results of tests of cold drawn bars of steel No. 9, 0.75 inchby 0.5 inch cross-section, in Krause fatigue testing machine. Rotatingbeam speed 7000 R. P. M. Cantilever loading It has further been foundthat the proportional limit of steels treated according to the inventionis lowered if the steel is subsequently cold-worked, and that the goodelastic properties may be restored by again applying the heat-treatmentdescribed herein. Inasmuch as many articles are fabricated by stepswhich involve cold-working of the metal (bending, spinning, etc.), thereare instances in which the method of the invention may advantageously beapplied to a partially or completely fabricated structure. For thispurpose, heating chambers, suitable for handling intricate or largearticles and apparatus, may easily be constructed for operation at thelow temperatures of this invention.

The application of the method of the invention to cold-rolledcorrosion-resistant austenitic chr'omium steel provides a new materialhaving improved properties which will permit designers to use higherunit stresses in the design of structures, and to use such higher unitstresses with full confidence in the dependability of the steel.

I am aware that it was proposed in British Patent 333,237 to enhance thecorrosion resistance of polished austenitic chromium-nickel steels byheating such steels within the temperature range of 100 to 400 C. for atime unspecified. The heat treatment of cold worked steels in accordancewith my invention has no detectable effect on the corrosion resistanceof the steels. For instance the results of immersing samples of steelNo. 1 (Table A) in boiling 65% nitric acid for three successive 48 hourperiods, were as appear- Prior workers have also observed a slightincrease in yield stress by a heat treatment of cold worked austeniticchromium nickel steels at temperatures between 300 and 450 C.(Monypenny, Stainless Iron and Steel, 2nd ed. page 188) and between 800and 1100 F. (P. D. Ffield Patents 2,080,367 and 2,080,368). There hasheretofore been no knowledge that, in the absence of precipitatablecompounds, the elastic properties of austenitic type chromium steels maybe valuably enhanced as disclosed herein.

I'claim:

1. Method of producing a steel structural member having throughout inboth tension and compression a proportional limit above 45,000 poundsper square inch and a, yield strength (at 0.2% permanent set) above160,000 pounds per square inch, which comprises cold rolling anaustenitic type chromium steel member until it has lost between 30% and60% of its original thickness, such steel member having the composition:15% to 19% chromium and 5% to 8% nickel, the sum of the chromium andnickel being between 21% and 26%, 0.01% to 0.2% each of carbon andnitrogen, 0.3% to 3% manganese, 0.08% to 0.8% silicon, remainder iron;and heating such cold rolled member at a temperature between 250 and 550F. for a time not over hours but at least long enough to satisfy theformula wherein H is the time in hours and T is the temperature on theFahrenheit scale.

2. Method of producing a steel structural member having throughout inboth tension and compression a proportional limit above 45,000 poundsper square inch and a yield strength (at 0.2% permanent set) between150,000 and 225,- 000 pounds per square inch, which comprises coldrolling an austenitic type chromium steel member until it has lostbetween 25% and 60% of its original thickness, such steel member havingthe composition: 15% to 19% chromium and 5% to 8% nickel, the sum of thechromium and nickel being between 21% and 26%, 0.01% to 0.2% each ofcarbon and nitrogen, 0.3% to 3% manganese, 0.8% to 0.8% silicon,remainder iron; and heating such cold rolled member at a temperaturebetween 250 and 550 F. for a time not over 100 hours but at least longenough to satisfy the formula (H) 10- wherein H is the time in hours andT is the temperature on the Fahrenheit scale.

3. A method of producing a steel structural member having throughout inboth tension and compression a proportional limit at least 30,000 poundsper square inch and a yield strength (at 0.2% permanent set) at least120,000 pounds per square inch, which comprises cold rolling anaustenitic type chromium steel member until it has lost between 10% and65% of its original thickness, such member having the composition: 12%to 25% chromium, at least one element selected from the group consistingof nickel and manganese in an aggregate percentage between 4% and 16%,0.01% to 0.3% carbon, 0.01% to 0.3% nitrogen, remainder principallyiron; and heating such cold rolled member at a temperature between 200and 625 F. for a time at least long enough to satisfy the formula"('H)10- =83 wherein H is the time in hours and T is the temperature onthe Fahrenheit scale.

4. Method of imparting improved resistance to tensile and compressivestresses and to fatigue, to a structure fabricated from a plurality ofcold rolled and otherwise cold worked structural members composed ofaustenitic type chromium steel which comprises heating such fabricatedarticle at a temperature between 200 F. and 550 F. for a time not over200 hours but at least long enough to satisfy the formula (H) 10 =83wherein H is the time in hours and T is the temperature on theFahrenheit scale.

5. Method of improving the elastic properties of cold worked austenitictype steel of the kind containing in the neighborhood of 18% chromiumand 8% nickel which comprises heating such steel at a temperature in theneighborhood of 400 F, for at least 8 hours but not so long assignificantly to impair the surface or to alter its corrosion resistanceor to harden the steel or to produce 'visually apparent precipitation ofcompounds.

6. Method of imparting improved resistance to tensile and compressivestresses and to fatigue.

to a cold rolled structural member composed of austenitic type chromiumsteel which comprises heating such cold rolled member at a. temperaturebetween 250 F. and 550 F. for a time not over 100 hours but at leastlong enough to satis- 1y the formula (H) 10- =150 wherein H is the timein hours and T is the temperature on the Fahrenheit scale.

RUSSEIL FRANKS.

Patent No. 2,558,799-

CERTIFICATE OF CORRECHON.

September 26, 191414..

RUSSELL FRANKS.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 5,second column, line 36, claim 2, for "0.8% to 0.8%" read "0.08% to'0.8%"; and that the said Letters Patent should be read with thiscorrection therein that the same may conform to therecord of the case inthe Patent Office. I

Signed and sealed this 28th day of November, A D. 191414.

Leslie Frazer (Seal) Acting Commissioner of Patents.

